Known in the state of the art for determining fill level and other process variables of a medium are so called oscillatory forks and single-rods. Used, in such case, is the fact that the characterizing variables (oscillation amplitude, resonance frequency, phase as a function of frequency) of the oscillations of the oscillatable unit are a function of contact with the medium and also a function of properties of the medium. Thus, for example, the frequency or the amplitude of the oscillations decreases, when the medium reaches, and at least partially covers, the oscillatable unit. Therefore, from the reduction of the oscillation frequency or the amplitude, it can be deduced that the medium has reached a fill level dependent on the embodiment and the mounting position of the apparatus. Furthermore, the oscillation frequency is also dependent, for example, on the viscosity and the density of the medium. In the case of embodiment in the form of an oscillatory fork, such measuring devices include at least two spades, or fork tines, which are mounted on a membrane and which oscillate with counter, or opposite, phase, relative to one another. This measuring principle is universally applicable for liquids and bulk goods. Through the spades, it is, however, not possible to provide a flush-mounted measuring device, since such measuring devices always have at least two spades, which register the medium.
Important in the case of mechanically oscillatable systems is, in general, that no oscillatory energy is lost, e.g. via coupling with the container. An energy loss is usually connected with a loss of applicability, or with defective measurements, or, in the application as switch, with erroneous switchings. At the same time, the energy loss can seldom be compensated by a higher driving power, since the described measuring devices, which, most often, are applied as switches, are usually operated with a small, and limited, energy supply.
For as broadly applicable measurements as possible, it is, furthermore, advantageous, when the effective mass of the mechanically oscillatable unit is as small as possible and the effective area, which comes in contact with the medium, is as large as possible. Against these considerations, however, is the problem, that the oscillatable unit is, preferably, introduced into the medium-containing container through bores which are as small as possible.
So-called membrane oscillators, or sensors, which work with the fundamental bending mode of the membrane, find only very limited applicability in measurements technology, since, in such case, there are always mechanical couplings of the membrane with the process connection. Therefore, such measuring devices can only be used in very limited cases for liquids and bulk goods. Advantageous, however, is that such vibratory limit switches, because they involve only a single membrane, enable a flushly mounting construction. This facilitates cleaning of the container/tank and brings advantages for process equipment in the foods industry, as well as in the production of chemicals and pharmaceuticals, etc.